This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device of the xe2x80x9cdivided alignments typexe2x80x9d, generally termed as multi-domain alignment (particularly, multi-domain-vertical-alignment) in which, by aligning the liquid crystal molecules differently in each domain within a single pixel, the visual-angle characteristics of the respective domains compensate for each other to provide a wide viewing-angle characteristic.
Widely known examples of liquid crystal display devices include those of the twisted nematic (TN) type and those which employ electrically-controlled birefringence (ECB). However, a problem with these conventional devices is that since the alignments of the liquid crystal molecules aligning under application of a voltage are uniform within a pixel, tonality differs depending upon the angle of view. A technique (multi-domain alignment) through which the directions of alignment of liquid crystal molecules in a single pixel are made to differ is available as a method of improving upon the visual-angle characteristic. With a liquid crystal device of this kind, the visual-angle characteristics of the multi-domains compensate for each other, as a result of which the characteristic is improved.
Multi-domain alignment methods are described in the specifications of Japanese Patent Kokai Publication JP-A-Nos. 7-318940, 8-292423, 9-80399, 9-304757 and 9-21913. These examples of the prior art place surrounding walls about a pixel and regulate the alignment of the wall surfaces to thereby realize an alignment that is symmetrical with respect to an axis perpendicular to a plate (substrate) at the center of the area surrounded by the walls. Multi-domain alignment is achieved as a result. Alternatively, protruding and recessed portions having axial symmetry with respect to the above-mentioned axis of symmetry are formed to correspond to the pixel, whereby similar multi-domain alignment is achieved. The art set forth in the specification of Japanese Patent Kokai Publication JP-A-8-292423 will be described with reference to FIG. 6. FIG. 6 is a sectional view showing one pixel of a conventional liquid crystal display device. As shown in FIG. 6, walls 23, 24 each comprising a resist or the like are formed on a plate 1 so as to surround a pixel electrode 22, and a recessed portion 25 consisting of a resist film is formed between the walls 23 and 24. A counterelectrode 26 is provided on a plate 2 on the opposite side of the device. and a projecting portion 27 is formed on the counterelectrode 26. The plates 1, 2 are arranged to oppose each other in such a manner that the recessed and projecting portions 25, 26 will have common axes of symmetry. If the gap between the plates 1, 2 is filled with a mixture of at least liquid crystal and a hardening resin and the liquid crystal and hardening resin are caused to undergo phase separation, a liquid crystal area will develop in such a manner that the liquid crystal precipitates in the recess 25 or surrounds the protrusion 27. When this occurs, the liquid crystal molecules in the vicinity of the recess 25 or in the vicinity of the protrusion 27 become oriented with axial symmetry, such as in radiating form or in the form of concentric circles, with the axis being perpendicular to the plates.
In the course of investigations toward the present invention, various problems have been encountered. Particularly a number of problems arise with the example of the prior art described above.
A first problem is that distribution of spacers (spheres etc.) generally used to maintain the panel gap between the plates is inappropriate. The reason for this is that the presence of spacers in the pixel areas tends to provide nuclei resulting in poor liquid crystal alignment, thereby greatly degrading the display characteristic. In contrast, if the spacers would be provided at portions where there are no pixels in order to avoid the problem of poor alignment, this increases the number of process steps.
A second problem is the requirement of process steps for mixing the hardening resin with the liquid crystal and causing phase separation and curing after the panel is filled. As a result, process load for forming the liquid crystal alignment is great.
Accordingly, an object of the present invention is to provide a multi-domain alignment liquid crystal display device in which regulation of the alignment of liquid crystal molecules is carried out through a simple process and panel gap can be maintained in stable fashion.
According to a first aspect of the present invention, there is provided a novel active-matrix liquid crystal display device. The device comprises generally first and second transparent insulating plates arranged to oppose each other, the first plate having disposed thereon a plurality of scanning lines and a plurality of signal lines, thin-film transistors provided in the vicinity of intersections between the scanning lines and signal lines, and pixel electrodes connected to the thin-film transistors, the second plate having a black matrix provided with openings at areas that oppose the pixel electrodes, a color layer and counterelectrodes provided so as to oppose the pixel electrodes, a liquid crystal sandwiched between the opposing first and second plates being controlled by voltage impressed across the pixel electrodes and counterelectrodes. Further an orientation layer is provided on the pixel electrodes of the first plate via an insulating film, the orientation layer being formed into a curved surface and causing molecules of the liquid crystal to become oriented in a direction normal to the curved surface of the orientation layer, and columnar spacers for regulating panel gap are provided between the two opposing plates. Particularly, each pixel is provided with one spacer.
In the first aspect of present invention, each columnar spacer has an end portion on one side thereof that preferably is disposed approximately at the center of the orientation layer formed on the first plate.
In a case where the orientation layer formed on the first plate defines a cavity recessed toward the side of the first plate in a cross section taken along a normal to the plate, the diameter of the columnar spacer becomes progressively smaller in the direction toward the second plate.
In a case where the orientation layer formed on the first plate defines a protrusion directed toward the side of the second plate in a cross section taken along a normal to the plate, the diameter of the columnar spacer becomes progressively larger in the direction toward the second plate.
According to a second aspect of the present invention, there is provided an active-matrix liquid crystal display device generally comprising first and second transparent insulating plates arranged to oppose each other, the first plate having disposed thereon a plurality of scanning lines and a plurality of signal lines, thin-film transistors provided in the vicinity of intersections between the scanning lines and signal lines, and pixel electrodes connected to the thin-film transistors, the second plate having a black matrix provided with openings at areas that oppose the pixel electrodes, a color layer and counterelectrodes provided so as to oppose the pixel electrodes, a liquid crystal sandwiched between the opposing first and second plates being controlled by voltage impressed across the pixel electrodes and counterelectrodes. Further, the pixel electrodes on the first plate and an orientation layer formed on the pixel electrodes define curved surfaces, and columnar spacers for regulating panel gap are provided between the two opposing plates.
In the second aspect of the present invention, the alignment (orientation) layer is formed, e.g., by oblique vapor deposition of SiO, and molecules of the liquid crystal are oriented substantially at right angles to the plane of the plate.
Each of the columnar spacers has an end portion on one side thereof that preferably is disposed approximately at the center of the pixel electrode formed on the first plate.
In a case where the pixel electrode formed on the first plate defines a cavity recessed toward the first plate in a cross section taken along a normal to the plate, the diameter of the columnar spacer becomes progressively larger (or smaller) in the direction toward the second plate.
In a case where the pixel electrode formed on the first plate defines a protrusion directed toward the second plate in a cross section taken along a normal to the plate, the diameter of the columnar spacer becomes progressively smaller (or larger) in the direction toward the second plate.
According to a third aspect, there is provided a multi-domain alignment active-matrix liquid crystal display device comprising; first and second transparent plates arranged to oppose each other; a liquid crystal being sandwiched between the first and second plates, and pixel electrodes disposed on one of the plates and counterelectrodes disposed on the other of the plates and adapted to apply voltage to the liquid crystal across the pixel electrodes and the counterelectrodes;
wherein an orientation layer is provided on each pixel electrode of one of said plates via an insulating film,
wherein the orientation layer is formed into a curved or slanted surface so as to orient molecules of the liquid crystal in a direction normal to the curved or slanted surface of said orientation layer, and
wherein columnar spacers are provided between the two opposing plates for regulating a panel gap between the plates.
The columnar spacers are disposed approximately at a center of the orientation layer on a pixel. The orientation layer defines a cavity recessed toward one of said plates. The columnar spacer has a side wall adapted to assist alignment of the liquid crystal molecules oriented by the orientation layer to secure multi-domain alignment thereof. The orientation layer defines a protrusion or recess directed toward one of said plates.
According to a fourth aspect of the present invention, there is provided a multi-domain alignment active-matrix liquid crystal display device comprising like components as in the third aspect, provided that an orientation layer is provided on each pixel electrode of one of the plates,
wherein the orientation layer and the pixel electrode are formed into a curved or slanted surface;
wherein columnar spacers are provided between the two opposing plates for regulating a panel gap between the plates.
According to a fifth aspect, there is provided a multi-domain alignment active-matrix liquid crystal display device comprising the components as in the third aspect, provided that an orientation layer is provided at least on each pixel electrode disposed on one of the plates, and
that columnar spacers are provided between the two opposing plates for regulating a panel gap between the plates.
The columnar spacers have a side wall adapted to pre-align molecules of the liquid crystal surrounding each of the columnar spacers centering thereat. The columnar spacers have a diameter varying along its axis. The columnar spacers have a diameter decreasing or increasing toward one end thereof.
The side wall is adapted to pre-align molecules of the liquid crystal, e.g., substantially parallel to the side wall. The orientation layer is formed into a curved or slanted surface so as to orient molecules of the liquid crystal in a defined direction normal to the curved or slanted surface of the orientation layer.
The curved or slanted surface is formed into a recess or protrusion.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.